Long history of embarrassingly premature triumphalism.
“[We are] probably nearing the limit of all we can know
about astronomy.” – Simon Newcomb, 1888
“The more important fundamental laws and facts of physical science have all been discovered.”
– Albert Michelson, 1894
“Physics as we know it will be over in six months.”
– Max Born, 1928
There is a 50% chance that “we would find a complete unified theory of everything by the end of the century.”
– Stephen Hawking, 1980

The Core Theory in more detail:
Quantum Mechanics
Think of “configurations,”
e.g. the location x of a particle.
Assign a complex number to
every possible configuration.
That describes a quantum state: a “wave function” Y(x)
that lives in a very-high-dimensional Hilbert space.
Schrodinger evolution equation:
x
x
Y(x)

(Some) Observables are Quantized
Standard example: Simple Harmonic Oscillator.
Particle moving in a potential ,
where x is the position and w is the frequency.
Energy is quantized
into discrete levels:

Quantum Field Theory
QFT is not a successor/alternative to QM; it’s just
a particular QM model, with a particular Hamiltonian.
Namely: “configurations” are “values of (relativistic)
fields throughout space.” E.g. f(x).
The quantum state (wave function) is a complex
amplitude for each possible field configuration, Y[f(x)].
Examples: electromagnetic field, electron field,
top quark field, gravitational field (metric), etc.

Think of your theory as only describing energies below
the ultraviolet cutoff scale L.
I.e., only include wavelengths longer than 1/L.
Result is an effective field theory below L.
Effective Field Theory

Both the field f and the cutoff L have units of energy,
and the Lagrangian governing interactions is (energy)4.
So schematically we have:
Higher-order terms are negligible at low energy (<< L).
Only a finite number of relevant/marginal interactions.
… but only a finite number of terms matter
“relevant”
“marginal”
“irrelevant”

“We haven’t quantized gravity,” but I’m treating
gravity like a perfectly ordinary effective field theory.
Because it is – as long as gravity is weak (far from
black holes, Big Bang, etc.).
In terms of curvature parameter R, interactions look like
Here on Earth, 1st term is 1050 times bigger than 2nd.
Quantum Gravity?

Constraints on new particles
As-yet-undiscovered
particles must be either:
very weakly interacting,
too heavy to create, or
too short-lived to detect.
In any of those cases, the new particle would
be irrelevant to our everyday lives.

To be relevant to everyday physics, any new forces
must interact with protons, neutrons, electrons,
and/or photons.
Experiments are ongoing (torsion balances) to
search for new, weak, long-range forces.
Two ways to hide:
weak interactions, or
very short ranges.
Constraints on new forces